The invention relates to a process for hydrogenating carbon-carbon double bonds of an unsaturated polymer by adding to the unsaturated polymer (1) a reducing agent selected from the group comprising hydrazines and hydrazine-releasing compounds, (2) an oxidising compound and (3) a catalyst.
A similar process is known from U.S. Pat. No. 4,452,950. This patent discloses that the reduction of carbon-carbon double bonds of an unsaturated polymer, with the polymer having been brought into the latex form, is carried out in the presence of a metal ion initiator. The metal ion initiator is a metal compound that reacts with hydrazine and is reduced by the hydrazine to a lower valence state. Examples of suitable metal ion initiators are Cu and Fe sulphates. After all reactants are added, the mixture is heated to the reflux temperature of the reaction mixture.
A drawback of this process is that the polymer is crosslinked early in the hydrogenation process and that heavy metals are left behind in the polymer after it is worked up.
The present invention aims to eliminate these drawbacks.
To that end, the invention provides a process in which the catalyst contains an element from group 13 of the Periodic Table of the Elements.
The Periodic Table of the Elements as used in the present application is published on the inside of the cover of the Handbook of Chemistry and Physics, 67th edition, 1986-1987 in accordance with the latest IUPAC nomenclature.
Furthermore, the process of the present invention presents the advantage that the reducing agent and oxidising compound may be present in equimolar amounts to at the most a minor excess relative to the double bonds of the unsaturated polymer to be reduced.
Also, the reaction is catalysed so well that a heating step may be omitted so that the hydrogenation process is much simplified.
Crosslinking of the latex during the hydrogenation process as described in U.S. Pat. No. 4,452,950 is mentioned in U.S. Pat. No. 5,039,737 and U.S. Pat. No. 5,442,009. Both patents disclose a process for breaking up the gel structures through post-treatment with ozone.
From application WO 91-A-06579 it is known to carry out a hydrogenation in the absence of a catalyst. However, this process employs a high hydrazine-to-unsaturated-polymer molar ratio in order to obtain a non-crosslinked polymer. Excess hydrazine subsequently needs to be worked up and destroyed, which from an economics point of view is costly and from an environmental point of view is unacceptable.
The unsaturated polymers that can be hydrogenated via the process of the present invention are composed of 5-100% by weight of a conjugated diene-monomer unit and 95-0% by weight of a vinyl-unsaturated monomer unit. Specific examples of conjugated diene-monomer units are 1,3-butadiene, 2,3-dimethyl butadiene, isoprene and 1,3-pentadiene. Specific examples of vinyl-unsaturated monomer units are nitriles (for example acrylonitrile and methacrylonitrile), vinyl aromatic hydrocarbons (for example styrene, (o-, m-, p-) alkylstyrenes and divinylbenzene) dialkenyl aromatics (for example diisopropenyl benzene), unsaturated carboxylic acids and esters thereof (for example acrylic acid, methacrylic acid, crotonic acid, itaconic acid, maleic acid, methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate and methyl methacrylate), vinylpyridine and vinyl esters (for example vinyl acetate).
The unsaturated polymers may be prepared in different manners for example through emulsion polymerisation, solution polymerisation and bulk polymerisation.
Specific examples of suitable unsaturated polymers are polybutadiene, polyisoprene, styrene-butadiene copolymers (SBR), acrylonitrile-butadiene copolymers (NBR), natural rubber, butadiene-isoprene rubber and terpolymers of butadiene, acrylonitrile and butylacrylate or acrylic acid.
Most preferably NBR is used.
The unsaturated polymers hydrogenated via the process of the present invention are characterised in that their backbone chain contains carbon-carbon double bonds that have an adverse effect on the polymer""s thermal, photochemical and oxidative stability.
During the hydrogenation, the unsaturated polymers preferably are present in the latex form. The latex form is an aqueous emulsion of polymer, in which sundry additives for example soap and stabilisers may be present. A description of the latex form which is suitable for the hydrogenation of unsaturated polymers via the process of the invention is given in for example U.S. Pat. No. 5,442,009.
The latex of the unsaturated polymer may be hydrogenated as such. The polymer concentration in the latex is in the range of from 1 to 70% by weight, preferably between 5 and 40% wt.
The reducing agent is selected from the group comprising hydrazines and hydrazine-releasing compounds for example hydrazine hydrates, hydrazine acetate, hydrazine sulphate and hydrazine hydrochloride. If the unsaturated polymer is hydrogenated in latex, use is preferably made of hydrazine and hydrazine hydrate. Alternative hydrazine sources may be used if hydrogenation is effected in solution or in the melt and if the alternative hydrazine sources do not adversely effect the stability of the latex.
Latices formed from, for example, non-ionic soaps may be used in combination with hydrazine, hydrazine hydrate, hydrazine hydrochloride and hydrazine sulphate.
The hydrazine or hydrazine releasing compound preferably is present in a molar ratio of from 0.1:1 to 100:1 relative to these carbon-carbon double bonds. Preferably, this ratio lies between 0.8:1 and 5:1, most preferably between 0.9:1 and 2:1.
Oxidising compounds that are suitable for the process of the invention are for example air, oxygen, ozone, peroxides, hydroperoxides, iodine, iodates, hypochlorite and similar compounds. Particularly suitable oxidising compounds are selected from the group consisting of peroxides and hydroperoxides. Most preferably, use is made of hydrogen peroxide.
The oxidising compound is preferably present at a molar ratio of 0.1:1 to 100:1 relative to the carbon-carbon double bonds. More preferably, this ratio is between 0.8:1 and 5:1, most preferably between 0.9:1 and 2:1.
It is preferred for the catalyst be a compound which contains boron (B). Examples of preferred B-containing catalysts are compounds of the general formula 
where X, Y and Z are chosen independently of one another from the group comprising R, xe2x95x90O, OR, OOR, NR2, SR, PR2, OC(xe2x95x90O)R and halogen atoms, where R is a H atom or an alkyl, aryl or cycloalkyl group having 1-20 carbon atoms, or a hydrocarbon group having 1-20 C atoms and containing a heteroatom from groups 14, 15, 16 and 17 of the Periodic Table of the Elements, or a polymerchain containing one or more polar groups;
wherein X and Z and optionally Y may form a bridge;
L is an electron-donating ligand, which may be bonded to either X, Y or Z; n=0, 1 or 2.
Examples of suitable electron-donating ligands are for example water, an alcohol, pyridine, bipyridine, triazine, pyrrole compound, an imidazole compound, a pyrazole compound, a pyrimidine compound and a pyrazine compound, an ester, ether, a furan, tetrahydrofuran, pyrans, dioxan, phosphine, phosphide, phosphate, a thio compound or a polymer, for example a polyvinylalcohol or polyethyleneoxide.
Salts of abovementioned boron-containing compounds might also be applied.
Preferably L is chosen from the group comprising diols, polyvinylalcohols and polyethyleneoxide.
It is preferred for the catalyst to be chosen from the group consisting of borates, peroxiborates and boric acid (H3BO3). More preferably, the catalyst is boric acid. It is most preferred that boric acid is used in combination with a polyvinylalcohol.
The catalyst of the present invention may be combined with the oxidising compound so that less or no oxidising compound needs to be added separately. An example of a catalyst that also has an oxidising effect is peroxiborate.
Another preferred embodiment of the present invention is addition of boric acid to the reaction mixture together with the peroxide or hydroperoxide.
The molar ratio of the catalyst to the carbon-carbon double bonds of the unsaturated polymer is between 1:1000 and 10:1. The ratio preferably is between 1:50 and 1:2.
The order in which the compounds are added for the hydrogenation reaction may in principle be random. Preferably, however, the oxidising compound is added last and in such a way that the concentration of the oxidising compound remains low during the hydrogenation reaction.
The degree of hydrogenation is the percentage of carbon-carbon double bonds that is saturated after the hydrogenation reaction relative to the initial amount of double bonds. The process of the present invention provides polymers that as a rule have a degree of hydrogenation higher than 60%. Preferably, the degree of hydrogenation is higher than 80%. The process is eminently suitable for preparing polymers having a degree of hydrogenation higher than 90%.
The hydrogenation reaction temperature normally is between 0xc2x0 and 250xc2x0 C. The temperature preferably is between 20xc2x0 and 100xc2x0 C. A reaction temperature between 40xc2x0 and 80xc2x0 C. is particularly preferable. Most preferable is a reaction temperature between 60xc2x0 C. and 80xc2x0 C., which results in a high catalyst activity, so that low catalyst concentrations can be used.
During hydrogenation in a latex, a minor amount of solvent for the unsaturated polymer may be present. In that case, the amount of solvent may vary between 0 and 20% wt (solvent relative to polymer).
The hydrogenation time normally is between 30 minute and 24 hours. The hydrogenation time preferably is between 1 hour and 12 hours.
The process is illustrated by the following examples without being limited thereto.
In almost all cases the degree of hydrogenation was determined with the aid of 1H-NMR in a Bruker 200 MHz instrument. The measurement was conducted on polymer precipitated in isopropanol, that was after precipitation dried with filter paper and immediately dissolved in deuterated chloroform. The degree of hydrogenation was calculated from the ratio of the integrals of the olefinic protons, which are between 5.2 and 5.6 ppm, and the proton adjacent to the nitrile group, being between 2.35 and 2.65 ppm.
In some cases (expressly indicated) the degree of hydrogenation was determined by Raman spectroscopy (using a Spectrum 2000 NIR FT Raman instrument).
The Cxe2x95x90C stretch vibrations of the vinyl, cis and trans bonds can be seen in the Raman spectrum between 1600 and 1700 cmxe2x88x921. The Cxe2x89xa1N stretch vibration can be seen at 2240 cmxe2x88x921. The peak height ratio between the Cxe2x95x90C stretch and the Cxe2x89xa1N stretch relative to the blank was used for determining the degree of hydrogenation, it being assumed that the stretch vibration of the nitrile group does not change significantly as a result of the unsaturated polymer being hydrogenated.